The vertebrate skeleton contains more than 200 bones, each with its own unique shape, size and function. Postnatally, bones remodel in response to the muscle forces they encounter. So bed rest, for example, causes bone thinning. Now, on , Amnon Sharir, Elazar Zelzer and colleagues report that muscle force also regulates bone shaping during embryogenesis in mice. Using micro-computed tomography scans of embryonic long bones, the researchers identify a novel developmental programme that, through asymmetric mineral deposition and transient cortical thickening, regulates the specific circumferential shape of each bone. This programme of preferential bone growth, they report, ensures that each bone acquires an optimal load-bearing capacity. Moreover, the programme is regulated by intrauterine muscle contractions; in a mouse strain that lacks such contractions, the bones lose their stereotypical circumferential outline and are mechanically inferior. Thus, the researchers suggest, a reciprocal relationship between structure and mechanical load in utero determines the 3D morphology of developing bones.
